Haemophilus influenzas is an important human pathogen and is especially common as a cause of localized respiratory tract diseases. Adherence to the respiratory epithelium plays an important role in H. influenzae infection, and is influenced by a number of H. influenzae factors. At least three different groups of H. influenzae proteins, Hap, the Hia/Hsf proteins, and the HMW1/2 proteins, are currently recognized as essential virulence factor adhesins. All of these adhesive proteins belong to the type V secretion pathway, which includes the auto-transporter pathway and the two-partner secretion (IPS) pathway. HMW1/2-like proteins are expressed by the majority (~80%) of non-typable clinical isolates but are generally absent from typable strains. Our long-term goal is to elucidate the structural basis of the HMW adhesin secretion pathway, a IPS system, in H. influenzae. While recent studies made significant progress in understanding the functional attributes of this important secretion pathway, the structural attributes of the proteins employed by this secretion system are totally absent. The proposed studies will employ crystallographic and biochemical approaches to obtain first structures of protein components of this system and to characterize structure-based functional relationships. A detailed structural and functional understanding of the HMW1 secretion strategies employed by HMW1, HMW1B, and HMW1C proteins should provide insights into one of the major bacterial secretion mechanisms. Further, our experimental results may find application in the control of bacterial pathogenesis and the development of new agents for the control of infectious diseases. The specific aims are to: [1]. Determine the structural basis of HMW1B translocator formation and function. We will (i) produce the HMW1B membrane protein amenable to crystallization, (ii) crystallize and solve the HMW1B structure, and (iii) investigate HMW1B variants with altered translocator activity. [2]. Determine the structural basis of the HMW1 adhesin maturation / secretion mechanism. We will (i) characterize the HMW1 secretion domain, (ii) characterize the HMW1 C-terminal domain, and (iii) determine the crystal structure of HMW1C, a putative glycosyltransferase.